Mechanical Properties and Thermal Stability of Rice HuskAsh Filled Epoxy Foams

P. M. Stefani,1,2 V. Cyras,1 A. Tejeira Barchi,2 A. Vazquez1

1Institute of Material Science and Technology (INTEMA), University of Mar del Plata-National Research Council(CONICET), Av. Juan B. Justo 4302, (7600) Mar del Plata, Argentina2Departamento de Ingenierıa Civil, Facultad Regional Concepcion del Uruguay, Universidad Tecnologica Nacional,Ingeniero Pereira 676, (E3264BTD) Concepcion del Uruguay, Argentina

Received 29 December 2004; accepted 30 June 2005DOI 10.1002/app.23001Published online 11 January 2006 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: Mechanical properties and thermal stabilityof epoxy foams filled with white and black rice husk ashwere studied. Epoxy foams were prepared from a commer-cial system and filled with different amounts of both theashes (0, 6.8, 12.8, 18.0, and 22.7 wt %). The incorporation ofboth the ashes modified the final morphology of the foam,decreasing the average cell size and increasing the numberof cells per volume unit. For all filler percentages used, thespecific modulus and strength results showed that the whiteash is more effective as reinforcing agent than the black ash.

The initial degradation temperature was not affected by thecontent and type of ash used as the filler. The integralprocedure decomposition temperature, weight loss, andchar residue results were related to the ash type and atmo-sphere used in the thermogravimetric analysis. © 2006 WileyPeriodicals, Inc. J Appl Polym Sci 99: 2957–2965, 2006

Key words: epoxy foam; rice husk ash; mechanical proper-ties; thermal stability; filler

INTRODUCTION

Epoxy foams have been developed because of theirhigh mechanical properties, excellent adhesion tomany materials, chemical, and good heat stability.They can be obtained by using physical or chemicalblowing agents. In the physical foaming process, bub-bles are generated from evaporation of low boilingliquid, such as freons, during the exothermiccrosslinking reaction.1 Another alternative is the useof microspheres where the gas containing particles arefilled mechanically into the epoxy matrix (syntacticfoams).2 During the last years, industries have devel-oped various epoxy foam systems where the blowingagent is chemically generated simultaneously with thecrosslinking reaction. It is possible to produce foamswith very different properties by changing the amountof chemical blowing agent or the curing conditions.During the foaming process, two parallel reactionsoccur in these systems: the reaction between epoxyand amine groups and the reaction between blowingagent with amino hardeners releasing a gas. The gasacts as expanding agent and the system foams out.3

This is an easy and economic method for the epoxyfoam production. Some publications have shown thatthese foams can be used as matrices of compositematerials and they have a good adhesion to most ofthe studied substrates.4

It is well known that the mechanical properties ofepoxy matrix can be improved by the inclusion offillers, as a second phase, to the resin batch. Silica(SiO2) is one of the fillers mostly used in epoxy sys-tems. Various publications have reported that silicafiller improves the thermal stability, increases themodulus, and decreases the thermal expansion coeffi-cient of epoxy matrix.5–7 Rice husk is one of the mainlignocellulosic residues of the agricultural industryand a natural source for silica production. World riceproduction in 2002/03 was �582 million tons. As aconsequence, 145 million tons of husk residues wereproduced. An increasing application of rice husk inrice producing countries, such as Brazil and Argen-tina, is as fuel in the heat generation for drying riceoperations, due to its high calorific power. However,in the combustion process, only the organic part canbe burned, giving silica-rich ash as the residue.

Rice husk contains a considerable amount of SiO2

(�17 wt %), compared with other agricultural prod-uct.8 However, because of the biological origin of ricehusk, small variations in the silica content can beobserved for different rice varieties. Mansaray andGhaly9,10 reported the char yield for different ricehusk varieties obtained from pyrolisis in oxidativeatmosphere at 700°C. Lemont, ROK 14, CP4, and Pa

Correspondence to: P. M. Stefani (pmstefan@fi.mdp.edu.ar).Contract grant sponsor: Facultad Regional Concepcion del

Uruguay, Universidad Tecnologica Nacional, National Re-seach Council (CONICET, Argentina).

Contract grant sponsor: PICT-SECyt Argentina.

Journal of Applied Polymer Science, Vol. 99, 2957–2965 (2006)© 2006 Wiley Periodicals, Inc.

Photo varieties showed char yield of 16.25, 17.06,22.20, and 15.50 wt %, respectively.9,10 These solidresidues are formed mainly by SiO2 (�95 wt %) andtraces other oxides such as Al2O3, Fe2O3, CaO, MgO,Na2O, K2O, and P2O5 in very small amounts.11

Depending on the burning conditions, silica contentand unburned carbon amount, two ash types can beobtained: white rice husk ash (WRHA) and black ricehusk ash (BRHA). The WRHA, mainly composed byamorphous silica, is obtained from the complete py-rolysis of rice husk in air up to a temperature of700°C.11–13 WRHA is mainly obtained in the upperlayer of rice husk mound in direct contact with airduring combustion. On the other hand, in the innerlayer, where the atmosphere is poor in oxygen, incom-plete combustion takes place, giving ash with a highpercentage of carbon residues (BRHA). Normally, ifboth ash types are milled, they can be used as filler fordifferent thermoset, thermoplastic, and elastomerpolymers.14–18

In our previous article, a commercial epoxy foamsystem was characterized.3 The optimum epoxy-amine relationship and the foaming agent amounteffect on the final morphology, density, and mechan-ical properties were determined. However, the effectof filler addition on foaming process of epoxy systemshas not been analyzed yet.

The aim of this work is to analyze the effect onfoaming process, morphology, thermal stability, andmechanical properties of epoxy foams by incorporat-ing rice husk ash as filler to a foaming epoxy system.

EXPERIMENTAL

Materials

The chemical system used in this work was kindlysupplied by Ciba Geigy, Bassel. Epoxy resin (LY5054)was heated to 80°C and degassed by vacuum over-night before use. The siloxane (HY5054) and amine(DY5054) were used as received.

Rice husk (Don Juan INTA, variety) used in thepresent work was collected from the Cooperativa Ar-rocera Sarmiento, Concepcion del Uruguay, EntreRıos, Argentina.

Preparation of rice husk ash

White and black rice husk ashes (WRHA and BRHA)were obtained from combustion of rice husk in air andargon atmosphere, respectively, in a laboratory ovenat a heating rate of 5°C/min from room temperatureto 600°C. Each sample was held at 600°C for 2 h. Theashes were crushed using an analytic mill (IKA 50,Germany) for 20 min.

Preparation of epoxy foams

The chemical reactions during the foam formation aredescribed in our previous article.3 Basically, 100

weight part of epoxy resin, 35 weight part of amine,and 1 weight part of siloxane were placed in a plasticcontainer. This system were mixed with 0, 10, 20, 30,and 40 weight part of rice husk ash for obtaining ricehusk ash filled epoxy foams with different ash con-tents (0, 6.8, 12.8, 18.0, and 22.7 wt %). The reactivemixture was stirred for 10 min using a mechanicalstirrer, and then it was placed in an oil bath at 50°C forthe free foaming process. The T � 50°C was selectedbecause the foaming process occurs at slow rate andprevents the coalescence of bubbles during foamingprocess. The obtained foams were then transferredinto an oven at 130°C for 3 h to ensure completion ofthe reaction.

Testing methods

X-ray scattering measurements were carried out witha Philips Model PW 1830 X-ray, operating at a Co K�radiation and equipped with a rotative sample-holderdevice. Measurements were performed in an angularrange of 5–70 2� at room temperature and a scanningrate of 1°/min (RX).

Silica content in the rice husk ashes was determinedgravimetrically: 1 g of ash was treated with 10 mL ofconcentrate HCl, followed by the addition of 10 mL ofconcentrate HNO3. Then, the sample was boiled for afew minutes and treated with 25 mL of HClO4. Thesample was filtered off and washed repeatedly withHCl and water. The sample was burned until reachingconstant weight.

Density of the WRHA and BRHA were determinedby picnometry using water as solvent following theprocedure described in the ASTM D792–87 norm.

The apparent density of the epoxy foams was de-termined using 20 mm � 20 mm (diameter and height,respectively) cylindrical specimens.

Compressive test was performed with a ShimadzuUH 100kNC machine following the test procedureASTM D-1621–94. The samples test size was 20 mm� 20 mm (diameter and height, respectively). Thedisplacement was calculated from the reading ob-tained with a linear variable differential transducer(LVDT) previously calibrated.

The SEM photographs of the polished surface of thedifferent foams were taken with a scanning electronmicroscope Philips model SEM 505. Micrographs weremagnified and regions containing about 100 bubbleswere analyzed. This led to the determination of thesize distribution, as n (number of bubbles) versus D(diameter) for each sample. The average diameter ofthe bubble (DB) and void fraction (fv) were calculatedform eqs. (1) and (2), respectively.19

Db ��nD

�n(1)

2958 STEFANI ET AL.

fv � ��/4��nD2

AT(2)

where AT is the total area of analysis.Thermogravimetric analysis (TGA) was carried out

using a Shimadzu thermal analyzer (Japan). Testswere performed in dynamic mode to cover a widerange of thermal conditions under nitrogen or airatmosphere (flow rate 200 mL/min). TGA tests wereperformed in alumina crucibles (5.9 � 4.7 mm2) wheresamples were placed without any previous treatmentand experiments were run immediately. The sampleweight in all tests was approximately 7–10 mg. Tem-perature programs were run from 30 to 750°C at 20°C/min. Data obtained from TGA experiments weretransformed in ASCII format and analyzed using astatistics computer program. The initial decomposi-tion temperature (IDT) is the onset of weight losstaken at 1%. The integral procedure decompositiontemperature (IPDT) was calculated from20–22:

IPDT�°C� � A*K*�TF � TI� � TI (3)

A* � �S1 � S2�/�S1 � S2 � S3� (4)

K* � �S1 � S2�/S1 (5)

where A* is the area ratio of total experimental curvedivided by the total TGA thermogram; K* is the coef-ficient of A*; TI is the initial experimental temperature;and TF is the final experimental temperature. A rep-

resentation of S1, S2, and S3 for calculating A* and K*is shown in Figure 1.20–22

RESULTS AND DISCUSSION

Characterization of rice husk ashes

The densities of the WRHA and BRHA determined bypicnometry were 2250 and 1650 kg/m3, respectively.The normalized size distribution curves of WRHA andBRHA powder are shown in the Figure 2. The particlesshow an irregular form with a minimum size of �110nm and a maximum size of 8 �m. The particle averagesize of WRHA and BRHA were 2.5 and 2.9 �m, re-spectively.

Thermograms of the WRHA and BRHA in air arerepresented in Figure 3. The thermogram of WRHAdid not show weight loss, because the combustionprocess for obtaining the ash was carried out in airatmosphere and the organic part was previously re-moved. However, even in air atmosphere, some car-bon residue cannot be removed by oxidation becauseit is fixed with silica.12 The BRHA sample shows anIDT of 245°C and a maximum weight loss of 49.6% (at700°C), which represents the amount of total unfixedcarbon in the sample.11

The SiO2 contents obtained gravimetrically were 93.1and 41.9 wt % for WRHA and BRHA, respectively. Thedifferences between solid residues obtained by TGA andresults of silica content determined gravimetrically indi-cate that approximately a 7 wt % of other oxides arepresent in both ashes, as aforementioned in this article.

Figure 1 Schematic representation of S1, S2, and S3 for calculating A* and K*.

RICE HUSK ASH FILLED EPOXY FOAMS 2959

Figure 4 shows the X-ray diffraction patterns ofWRHA and BRHA. The pyrolysis conditions of ricehusks (temperature, time, and atmosphere) are impor-tant to define whether silica remains essentially amor-

phous, as in the case of WRHA and BRHA used in thiswork. WRHA shows a peak at about 2� � 23°, whichindicates an incipient crystallization to crystobalite.23

On the other hand, the peak at about 2� � 31° indi-

Figure 2 Size distribution of WRHA and BRHA.

Figure 3 TGA curves for the ashes in air (a) WRHA and (b) BRHA.

2960 STEFANI ET AL.

cates the presence of unburned carbon.24 Therefore, itwill be necessary to heat at higher temperatures or togive any acid pretreatment to remove the unburnedcarbon.12,25

Morphology

Figure 5 shows the micrographs obtained by SEM ofthe pure and filled epoxy foams with differentamounts of WRHA or BRHA. In all samples analyzed,bubbles are spherical with few contact points betweenthem, as it is characteristic in high density foams. Thevariation in the bubble size distribution and averagebubble size with the ash content is shown in Figures6(a) and 6(b). The average bubble size decreasesabruptly from 258 to 110 �m for the pure epoxy foamand the sample modified with 6.8 wt % of WRHA,respectively, remaining constant for higher percent-ages of WRHA [see Fig. 6(b)]. On the other hand,when BRHA is used, the average bubble size de-creases smoothly up to 155 �m for 22.7 wt % of ash[see Fig. 6(b)]. The incorporation of rice husk ash intothe basic system produces a nucleating effect duringfoaming process, increasing the number of bubblesper volume unit and decreasing its average size andthe width of distribution.

Density and mechanical properties

Table I shows the mechanical properties in compres-sion and density data for the pure and rice husk ashfilled epoxy foams with different type and ash con-

tents. The density of the foams is a function of voidsfraction and filler content.3 As the ashes have a higherdensity than epoxy matrix, its incorporation shouldcause a density increase in the final foam. This in-crease is more notable in the foams filled with WRHAdue to their higher density respect to BRHA. How-ever, the increase in density due to the incorporationof ash may partially be compensated by an increase inthe void fraction. As the results of Table I show, thiscompensation effect is observed in all foams, espe-cially in the samples filled with 12.8 wt % of bothashes. It showed a minimum in density in agreementwith the maximum void fraction determined.

To compare the effectiveness of filler on mechanicalproperties, the specific properties (property/density)were determined. As the results of Table I show, in theWRHA-filled samples the specific modulus andstrength notably increase with filler content. On theother hand, when BRHA was used as filler, only aslight improvement in the specific strength was ob-served for the higher percentage analyzed. This dif-ferent behavior can be explained by analyzing thevariables that normally affect the modulus and com-pressive strength in filled foams: voids fraction, cellsize, and filler content. The compression mechanicalproperties improve if the foams have a small cell size,low voids fraction, and high filler content.2,3,6,26–28

Independently, the mechanical behavior of the foamsimproves when the ash content increases, and thefoams filled with WRHA present a lower cell size andlower voids fraction than foams filled with BRHA,explaining their different behavior.

Figure 4 X-ray diffraction patterns (a) WRHA and (b) BRHA.

RICE HUSK ASH FILLED EPOXY FOAMS 2961

Figure 5 SEM microphotographies of epoxy foams with different ash contents (a) pure epoxy foam, (b) EF-WRHA10 (6.8 wt%), (c) EF-WRHA20 (12.8 wt %), (d) EF-WRHA30 (18 wt %), (e) EF-WRHA40 (22.7 wt %), (f) EF-BRHA10 (6.8 wt %), (g)EF-BRHA10 (12.8 wt %), (h) EF-BRHA30 (18 wt %), and (i) EF-BRHA10 (22.7 wt %).

2962 STEFANI ET AL.

Figure 7 shows the final forms of the pure epoxyfoam and filled with WRHA or BRHA after compres-sion test. In all the tests, the load increases continu-ously even after the critical compressive strength (Sc)has been attained. At the same time, the crosssectionof the sample increases and assumes a barrel-likeshape up to failure is reached. The deformation atbreak values varied between 50 and 60% and did notshow a tendency with the content and type of ash. Inthe pure epoxy foams, the barrel-like shape was main-tained after testing and small cracks were observed insome cases. On the other hand, in agreement with thefailure form described in the literature,1 all samplesfilled with WRHA and BRHA showed a brittle failureinvolving cracking along the incline planes.

Thermal stability

Figures 8(a) and 8(b) show the typical dynamic TGAcurves of pure epoxy foams and filled with 18 wt % of

BRHA or WRHA in air and nitrogen atmosphere. Thepatterns of the thermogravimetric curves of the epoxyfoam filled with WRHA and BRHA were similar to thepatterns of the pure epoxy foams, showing that thethermal degradation of foamed epoxies consists in oneor two independent stages under nitrogen or air at-mosphere, respectively.5,21,22 The thermal stability ofpure and ash-filled epoxy foams were analyzed bydetermination of IDT, maximum weight loss temper-ature (Tmax), and IPDT.

The thermal stability parameters of all the obtainedfoams are summarized in the Table II. IDT’s resultsshowed a difference between 60 and 66°C with atmo-sphere type (air or nitrogen), in agreement with otherworks reported in the literature.29 The incorporationof WRHA or BRHA did not significantly alter IDT andTmax of the pure epoxy foams in nitrogen atmosphere.On the other hand, the IPDT increased with the ashcontent (WRHA or BRHA) because their volatile frac-tion is reduced. IDT results in air did not significantlychange with the amount and type of ash. However,Tmax in the first stage of BRHA-filled epoxy foamsshifted toward lower temperatures, while in the foamsfilled with WRHA remained constant. As it was pre-

Figure 6 (a) Bubble size distribution of epoxy foams withdifferent ash contents. (—-) pure epoxy foam, (f) EF-BRHA10 (6.8 wt %), (Œ) EF-BRHA20 (12.8 wt %), (�) EF-BRHA30 (18 wt %), (�) EF-BRHA40 (22.7 wt %), (‚) EF-WRHA10 (6.8 wt %), (�) EF-WRHA20 (12.8 wt %), (E)EF-WRHA30 (18 wt %), and (�) EF-WRHA40 (22.7 wt %).(b) Variation of average size of bubble for the foams filledwith different ash contents (WRHA and BRHA).

TABLE IMechanical Properties, Density, and Voids Fraction of the Pure and Filled Epoxy Foams

Sample RHA (wt %) � (kg/m3) E (MPa) Sc (MPa) fV E/� (MPa m3/kg) Sc/� (MPa m3/kg�1)

EF 0 320 � 28 149.9 � 33.1 5.14 � 1.47 0.65 � 0.1 0.469 0.0160EF-WRHA10 6.8 313 � 23 171.1 � 11.6 5.30 � 0.75 0.68 � 0.2 0.547 0.0169EF-WRHA20 12.8 310 � 16 209.9 � 10.7 5.32 � 0.41 0.70 � 0.2 0.677 0.0171EF-WRHA30 18.0 339 � 24 249.2 � 24.6 6.28 � 1.04 0.68 � 0.1 0.735 0.0185EF-WRHA40 22.7 367 � 12 279.0 � 27.2 7.38 � 0.99 0.67 � 0.1 0.760 0.0200EF-BRHA10 6.8 302 � 29 134.1 � 32.7 5.10 � 1.12 0.69 � 0.2 0.444 0.0169EF-BRHA20 12.8 285 � 27 127.7 � 25.1 4.71 � 0.99 0.71 � 0.3 0.448 0.0165EF-BRHA30 18.0 313 � 26 162.1 � 28.3 5.48 � 0.89 0.69 � 0.1 0.518 0.0175EF-BRHA40 22.7 320 � 27 183.6 � 28 5.56 � 0.91 0.69 � 0.1 0.574 0.0174

Figure 7 Image of samples after compression test (a) pureepoxy foam, (b) BRHA filled epoxy foam, and (c) WRHAfilled epoxy foam.

RICE HUSK ASH FILLED EPOXY FOAMS 2963

viously indicated, Figure 8(a) shows in detail that theweight loss rate increases as a consequence of thethermal decomposition of BRHA inside this tempera-ture range. In the second stage, the oxidation of theresidue formed from the thermal decomposition of thematrix at 200–400°C occurs. Both ashes accelerate thethermal decomposition of the residue as shown by thelower values of Tmax (see Tmax2 in Table II). IPDTresults in air increased with the ash content (WRHA orBRHA) because their volatile fraction is reduced. Thesamples filled with WRHA showed a higher IPDTthan samples filled with BRHA. This behavior is aresult of higher silica content and consequently thehigher thermal stability of WRHA. A similar effectwas reported when other inert inorganic fillers wereadded to different polymers.5

The char yield of all the analyzed samples afterreaching 700°C is shown in the Table II. Black andwhite chars were obtained under nitrogen and air

atmospheres, respectively. For the pure epoxy foam, achar yield of 1.5 wt % was produced in air atmo-sphere. This solid residue may be attributed the for-mation of SiO2, from the oxidation of siloxane chainsegments present in a low amount in the chemicalstructure of the polymeric network.30 For the samesample, a char yield of 11.9 wt % was obtained undernitrogen atmosphere. In this case, the black color andthe higher solid residue percentage are attributed tothe formation of residual carbon.30

For the WRHA- or BRHA-filled epoxy foams, thechar yield increased proportionally with the content ofboth ashes. On the other hand, under air atmosphere,lower char yields were obtained in the samples filledwith BRHA. This difference is due to the higher con-tent of carbon in the BRHA, which is removed inoxidative atmosphere.

The solid residues percentage after reaching 700°Cin all the composites can be predicted by using asimple mixture rule from experimental values of charyield of pure component (ash and epoxy foam) ob-tained under the same conditions as follows.31

WT � fRHAWRHA � fEFWEF (6)

where WT is the predict weight residue percentage;WRHA and WEF are the weight percentage of rice huskand epoxy, respectively; and fRHA and fEF are theweight fraction of rice husk and epoxy, respectively.The experimental and predict char percentages weresimilar for all percentages analyzed, even while con-sidering the experimental error of the measurements(Table II). These results suggest that in the residue thecomponents behave as individual ones with no inter-action between them.

CONCLUSIONS

The incorporation of two types of rice husk ashes toepoxy foams modify their final morphology, decreas-ing their average cell size and increasing the numberof cells per volume unit. This indicates that rice huskash particles act as a nucleating agent on foamingprocess.

The mechanical properties of the epoxy foams im-prove especially with the incorporation of WRHA. Forexample, the specific modulus increases from 0.469 to0.760 MPa m3/kg for 0 and 22.7 wt % of WRHA. Thisimprovement is not observed in the samples filledwith BRHA.

The initial degradation temperature did not changewith the incorporation of ash. However, BRHA accel-erates the thermal decomposition during the initialstage in air atmosphere. In all the samples, the pres-ence of inorganic and thermally stable filler reducesthe amount of volatiles during the complete thermaldegradation process, as shown by the higher IPDTresults.

Figure 8 TGA curves of the epoxy foams. (—-) Pure epoxyfoam, EF, (���), EF-WRHA30 (18 wt %), and (——) EF-BRHA30 (18 wt %). (a) In air and (b) in nitrogen.

2964 STEFANI ET AL.

The final conclusion of this study is the valid use ofWRHA as reinforcing agent of epoxy foams. Thismeans the increase of the added value for rice husk,currently considered a waste product with no indus-trial reuse possibilities.

The authors thank Mr. Eliseo Britos for his technical assis-tance in the RX and TGA tests.

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TABLE IIThermal Stability Data from Thermogravimetric Analysis

Sample RHA (wt %) Atm. IDT (°C) Tmax1 (°C) Tmax2 (°C) IPDT (°C)Exp. char at 700°C

(wt %)Predict char at 700°C

(wt %)

EF 0 N2 322 389 548 11.9 11.9EF-WRHA10 6.8 N2 322 389 640 18.6 18.7EF-WRHA20 12.8 N2 321 390 726 23.7 24.7EF-WRHA30 18.0 N2 323 390 879 31.4 29.9EF-WRHA40 22.7 N2 323 389 1072 35.9 34.6EF-BRHA10 6.8 N2 323 389 668 20.2 17.4EF-BRHA20 12.8 N2 324 388 737 24.4 22.3EF-BRHA30 18.0 N2 323 390 824 26.8 26.5EF-BRHA40 22.7 N2 325 390 889 33.8 34.6

EF 0 Air 255 324 567 464 1.5 1.5EF-WRHA10 6.8 Air 257 325 560 511 8.4 8.3EF-WRHA20 12.8 Air 256 324 549 613 13.7 14.3EF-WRHA30 18.0 Air 257 326 548 684 18.4 19.5EF-WRHA40 22.7 Air 258 329 545 768 23.3 24.2EF-BRHA10 6.8 Air 257 321 565 504 4.5 4.8EF-BRHA20 12.8 Air 256 313 546 534 7.7 7.8EF-BRHA30 18.0 Air 256 314 545 595 12.3 10.4EF-BRHA40 22.7 Air 255 312 544 614 12.9 12.7

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